1. Field of the Invention
The present invention relates to an optical communication apparatus and an optical communication method, particularly to an optical communication apparatus and an optical communication method for transmitting and receiving information by a light beam emitted by a laser diode, an LED (Light Emitting Diode) or the like with optical fiber as a transmitting medium.
2. Description of Related Art
FIG. 8 is a block diagram for explaining an example of the constitution of a conventional optical communication apparatus. A laser diode (hereinafter, abbreviated as LD) 1 converts an electric signal into a laser beam having a corresponding intensity and transmits it via an optical fiber, not illustrated. Further, an LD drive unit 2 drives LD 1 in response to transmitted data (Tx. Data) inputted from a signal line 3 and an output signal from APC (Auto Power Control) 5. A portion of the laser beam emitted from LD 1 is incident on a photodiode 4 where the portion is converted into a corresponding electric signal.
APC 5 controls the LD drive unit 2 in response to the electric signal outputted from the photodiode 4 and a reference level (Tx. Ref) of the transmitted signal inputted via a signal line 6 such that the intensity of the laser beam emitted from LD 1 becomes a predetermined level. A laser beam transmitted via an optical fiber, not illustrated, is incident on a photodiode 8 and the photodiode 8 converts the transmitted laser beam into a corresponding electric signal. An amplifying unit 9 amplifies the electric signal outputted from the photodiode 8 with a predetermined gain and outputs the amplified electric signal as received data (Rx. Data) via a signal line 10.
Next, an explanation will be given of the operation of the conventional example.
The transmitted data is supplied to the LD drive unit 2 via the signal line 3. The LD drive unit 2 drives LD 1 in response to the transmitted data and the output signal from APC 5 whereby a laser beam is emitted. The laser beam emitted from LD 1 is transmitted to a counter side of communication, not illustrated, via an optical fiber, not illustrated.
A portion of the laser beam emitted from LD 1 is incident on the photodiode 4 and accordingly, an electric signal in correspondence with the intensity of the laser beam emitted from LD 1 is inputted to APC 5. APC 5 compares the electric signal outputted from the photodiode 4 with the reference level (Tx. Ref) of the transmitted signal inputted from the signal line 6 and controls the LD drive unit 2 such that a predetermined relationship is maintained therebetween (for example, the both are provided with the same value). As a result, the intensity of the laser beam emitted from LD 1 is always set to a predetermined level.
Further, the laser beam transmitted via an optical fiber, not illustrated, is photoelectrically converted into a corresponding electric signal by the photodiode 8 and amplified by the amplifying unit 9 with a predetermined gain and thereafter, the electric signal is outputted as received data via the signal line 10.
According to such a conventional optical communication apparatus, the intensity of the transmitted laser beam is always set to a predetermined level. The intensity of the laser beam is generally set with a transmission loss of a longest optical fiber in the system (system constituted by mutually connecting optical communication apparatuses) as a reference.
That is, when the laser beam is set to have an intensity capable of sufficiently dealing with the transmission loss at a longest optical fiber in the system, a sufficient intensity can be provided in all of portions of the system.
However, it is said that the life of a laser diode (LD) is inversely proportional to a square (a cube) of the intensity of an emitted laser beam. Therefore, when the intensity of the laser beam is set in compliance with a longest optical fiber in the system, the intensity becomes excessively large at other portions of the system and as a result, the life of LD is considerably shortened as a whole in the system.
Further, when there is a large variational width in respective transmission losses of optical fibers mutually connecting optical communication apparatuses constituting a system (when respective lengths of optical fibers differ considerably), a large variational width is similarly provided to intensities of laser beams inputted to receiving units of optical communication apparatuses. For example, in the case of LAN (Local Area Network) or the like, there is a large variational width substantially ranging from 1 m to 100 m in respect of lengths of optical fibers used and accordingly, a difference in transmission loss amounts to substantially 16 dB whereby a large variational difference is given to intensities of laser beams in correspondence thereto. Further, such a difference is especially significant in POF (Plastic Optical Fiber) having a large internal loss.
When there is such a large variational width in the intensities of inputted laser beams, in order to secure a constant error rate in respect to inputs of laser beams at any intensities, the dynamic ranges of the optical communication apparatuses need to be secured sufficiently widely and as a result, the design of the apparatuses becomes complicated and the fabrication cost of the apparatuses is increased.
Furthermore, when the problem of Eye Safe is considered, it is preferable to set the intensity of a laser beam as small as possible. However, when the intensity of a laser beam is set to a low value, according to a system having a large loss (for example, a system connected by POF or the like), the system conflicts with the problem of the dynamic range as described above which makes difficult the design of the system.
Hence, there is known a method (OFC; Optical Fiber Control) clearing the problem of Eye Safe in which when optical fibers are not connected to optical communication apparatuses, the nonconnected state is detected and laser beams are outputted intermittently by which an average value of the intensity over time is lowered.
However, according to such a method, a circuit for detecting the nonconnected state of an optical fiber and a circuit for outputting a laser beam intermittently are further needed whereby the cost of apparatus is increased.
The present invention has been carried out in view of the above-described situation whereby the design of an optical communication apparatus can be simplified, the fabrication cost can be reduced and the life of apparatus can be extended.
Further, according to the present invention, an optical communication apparatus can be operated stably and the problem of Eye Safe can be cleared even if the optical communication apparatus is used under any environment.